PROPOSAL OF EVALUATION METHODS OF THE OUTFLOW RATE OF BACKFILL MATERIALS FROM A GENTLY SLOPING DIKE COVERED WITH BLOCKS

Abstract

 In the case of coastal dikes and seawalls installed in extremely shallow waters or on sandy beaches, damage occurs from the base deformation due to frontal scour. Moreover, if the scour depth is deep enough, the damage occurs from backfill material outflow on the return of incident waves entering the body. Studies on the backfill material outflow from coastal dikes and seawalls is becoming increasingly important as the environment on coastal disaster deteriorates due to coastal erosion and global warming. Therefore, since there are many studies of the backfill material outflow on steeply sloping dikes and seawalls, we carried out a study on a gentle-sloping dike with concrete blocks covering the surface slope.

 First, hydraulic modelling experiments for gently sloping dikes covered with blocks revealed that the frontal scour does not occur significantly and backfill materials flow out through the gaps between front-covered blocks and, after the blocks had fallen, backfill materials continue to flow out until the slope of the backfill section becomes equal to the slope of the front beach. It was also confirmed that the outflow rate of backfill materials increases as the incident wave height and period increase, as the frontal water depth increases and as the median grain size of backfill materials decreases. Furthermore, a formula and a numerical simulation model for reproducing those phenomena were proposed, and good correlations with validation data were also confirmed.